Polymerase chain reaction (PCR) amplification of a portion of the genome of both rapidly growing mycobacteria and nocardia, followed by restriction fragment length polymorphism (RFLP) analysis of the amplification products, is being evaluated to assess the utility of these procedures for use in the diagnostic laboratory. The technique has already proven useful in providing preliminary identification of these organisms within a few days of organism isolation, as compared with the month or more required for conventional identification based on biochemical testing. In addition, these molecular procedures seem generally to allow more accurate discrimination among species and subspecies than is possible with biochemical testing. Our work with a portion of the genome for 16S ribosomal RNA has suggested the existence of a hitherto unrecognized Nocardia species. Some of this work is currently being prepared for publication.For the rapidly growing mycobacteria, results of the studies with this procedure have stimulated us to reassess the salt tolerance test, which is widely used to help with the phenotypic identification of these organisms. We have defined the optimal conditions for performing the salt tolerance test, and have shown that even under the best of circumstances it does not have the discriminating power of the molecular methods. A manuscript reporting these results has been published.